This invention relates to an improved instrument panel and an electroluminescent (EL) lamp therefor, and more particularly to an improved EL lamp of the type which is particularly suited for use in aircraft instrument panels. Even more particularly, this invention relates to an improved EL lamp having self-regulating means for maintaining the intensity or brightness of the lamp for its entire life span.
Circa World War II, aircraft cockpits customarily were equipped with instrument panels made of plastic, and having embedded therein a plurality of miniature incandescent bulbs for illuminating transluscent nomenclature formed on the face of the instrument panel. In more recent years aircraft panels have incorporated therein EL lamp devices which provide extremely uniform light as opposed, for example, to hot spots of light which often were produced in the older, incandescent-type instrument panel. Aircraft panels utilizing EL lamp illumination in general have much improved useful operating lives as compared to the incandescent-type instrument panels. This application is primarily concerned with aircraft panels which have incorporated therein hover EL lamps or panels for illuminating the instrument panel nomenclature.
Before proceeding further, it should be noted that electroluminescent lamps are often manufactured in flat, panel configurations, and for that reason are often referred to as either EL lamps or EL panels. Since the hereinafter described products are particularly suited for incorporation into aircraft instrument panels and the like, they will normally be referred to as EL lamps to avoid confusion with the panels (instrument) in which they are embedded or otherwise incorporated. EL lamps of the type which heretofore have been incorporated in instrument panels and the like are disclosed in, for example, U.S. Pat. Nos. 4,578,617 and 4,619,624, which patents are assigned to the same assignee as is this application.
Aircraft instrument panels of the type described are generally supplied with an alternating current (AC), typically in the range of 115 volts at 400 hertz (Hz). Moreover, this voltage supply customarily is readily adjustable optionally to supply an AC voltage ranging anywhere from, for example, 20 to 115 volts so that the intensity of the light emitted by an associated EL lamp can be reduced simply by reducing its operating voltage. However, in connection with the EL lamps described herein, there is a minimum threshold voltage which must be exceeded (e.g., in excess of 30 or 40 volts) before the lamp will begin to emit light.
Although more efficient and longer lasting than the old incandescent-type aircraft panels, instrument panels using EL lamps have the disadvantage that the brightness or light intensity of the respective EL lamps in the panel tends to decrease over the life of the lamp, notwithstanding the fact that its operating voltage and frequency are held constant over the same period of time. For example, in some cases conventional EL lamps may be capable of only half their initial brightness after only 1500 hours of use. For this reason, this so-called "brightness decay" has often discouraged the use of EL lamps for backlighting. Moreover, since this loss of brightness often limits the useful life of instrument panels of the type described, it is often necessary to require periodic replacement of instrument panels or their EL lamps. The replacement, in turn, creates new problems, particularly in the case of aircraft instrument panels, because the new panels or replacement lamps often emit more brightness or light intensity at the same voltage than did the "old" panels or lamps which they replaced and as a consequence this often results in distracting differences between the various instrument panels in a cockpit.
In an effort to solve this problem aircraft panels have in the past been manufactured with a special light sensing mechanism comprising a single EL lamp mounted in an instrument panel to face rearwardly, as contrasted to the remaining EL lamps which face forwardly of the panel to illuminate various nomenclature thereon. The single rearwardly facing lamp directed its light onto a photodetector which was connected to a common power supply which applied the exiting voltage for all the EL lamps in the panel, including the rearwardly facing "control" lamp. The problem with this solution, however, was that the intensity of the one, "control" lamp was not truly indicative of the intensity of each of the remaining lamps in the panel which in fact glowed with different intensities, thus producing undesirable brightness differences, particularly at low brightness levels.
It is an object of this invention, therefore, to provide an improved instrument panel of the type described which is illuminted by EL lamps, and which has incorporated therein means for maintaining substantially uniform light intensity of its lamps throughout the useful lives of the lamps.
Still another object of this invention is to provide for instrument panels of the type described an improved EL lamp and associated self-regulating means which will function automatically to adjust the operating voltage of the lamp, therefore to maintain a uniform light intensity in the EL panel.
A further object of this invention is to provide in improved EL lamp having a novel voltage control circuit therefor which will significantly prolong the useful life of the lamp, and which at the same time can be adjusted to maintain its light intensity at a predetermined value.
Still other objects of this invention will be apparent hereinafter from the specification and from the recital of the appended claims, particularly when read in conjunction with the accompanying drawings.